Question: Which of these numbers is prime? ${8,\ 9,\ 47,\ 69,\ 85}$
Answer: A prime number has exactly two factors: 1 and itself. The factors of 8 are 1, 2, 4, and 8. The factors of 9 are 1, 3, and 9. The factors of 47 are 1 and 47. The factors of 69 are 1, 3, 23, and 69. The factors of 85 are 1, 5, 17, and 85. Thus, 47 is a prime number.